For many purposes it is desirable to know when liquid reaches or falls below a certain predetermined level. When filling a tank, such as a tank for transporting liquified natural gas (LNG) at a cryogenic temperature, knowing precisely when the tank is filled to a certain level will prevent overfilling. In a cryogenic cooling system for superconductors, the level of fluid may be highly critical as the evaporation of the cryogenic coolant may result in normalization of superconducting circuits and consequential burning out of intricate and expensive apparatus.
Among the types of liquid level sensors available are capacitor sensors which respond to the difference between the dielectric constants of the liquid and the atmosphere and which, through appropriate circuitry, are linked to indicia which alert an operator of liquid level change and/or to relays to activate or deactivate associated apparatus as the liquid level changes. For example, an audible or visual alarm may be linked to the capacitor sensor so that workers filling an LNG tank will know that the tank is filled and cease filling the tank. In a superconducting system, the liquid level sensor may be linked to a switch which cuts power to superconducting circuitry should the cryogenic coolant fall below a certain level so that normalization of circuitry does not result in electrical damage.
Accuracy of liquid level sensing requires that the apparent liquid level at a sensor, such as a capacitor sensor, reflect the true liquid level in the tank. For accurate liquid level detection, it is desirable to eliminate various factors which could provide a capacitor sensor with a false apparent level.
In a ship's LNG tank which is being filled, the surface of the liquid is disturbed by the inflow of the liquid and conceivably by waves in the harbor. The resulting liquid surface movement in the tank might alternately expose the capacitor sensor to atmosphere when the true average liquid level is above the sensor and submerge the sensor when the true average liquid level is below the sensor. The term "atmosphere" herein is used broadly to include ambient atmosphere or an atmosphere which consists primarily of vapors of the liquid as will often be the case in a cryogenic tank. Air and most gases have dielectric constants of about 1. If other sensors, such as thermal sensors, are used to detect the liquid level, they are selected to respond to differences in physical properties of the "atmosphere" and the liquid, but not between various "atmospheres". To isolate liquid level sensors from such apparent fluid level changes, prior art devices have employed stillwells which, in their simplest form, may take the form of a tube open at both ends with the top end in communication with the atmosphere and the bottom end in communication with the liquid. The liquid level sensor, which is located in the stillwell, is substantially laterally isolated from the effects of wave motion or sloshing in the tank. Furthermore, it is necessary to prevent foreign matter that may enter the tank along with the cryogenic liquid from interferring with the liquid level sensor.
A potential cause of false readings by a liquid level capacitor sensor is bubbles which may accumulate between the plates thereof. Bubbles are particularly problematic with low boiling point liquids, especially cryogenic liquids, which are at equilibrium so that the inflow of heat from the environment results in the creation of vapor bubbles. Bubbles also result from the mechanical disturbances and the effects of pumping and fluid flow when a tank is being filled. Thus, the problem of bubbles is particularly acute at a time when it is most important to know the level in the tank. Bubbles trapped between capacitor plates and below the surface may lower the dielectric constant between the plates to give the capacitor sensor a misleadingly low capacitance. Alternatively, bubbles bridging the gap between capacitor plates may give the capacitor sensor a misleadingly high capacitance well after the liquid level has receded below the level of the capacitor sensor.
It would be desirable to provide liquid level sensing apparatus in which false readings resulting from bubble-sensor contact are substantially eliminated.
Accordingly, it is a primary object of this invention to provide liquid level sensing apparatus in which the sensor is shielded from bubbles.